ELECTRICAL QUANTITIES AND CIRCUITS cie igse physics
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1. There are positive and negative electric charges. Positive charges repel other positives and negative charges repel other negatives, while positives and negatives attract. 2. Charging objects by friction involves transferring electrons, giving the object a positive charge. 3. Electric fields are regions of force around charged objects, directed away from positive charges and toward negatives. 4. Conductors allow charge carriers like electrons to flow through easily, while insulators do not, making conductors better for electric currents.
ELECTRICAL QUANTITIES AND CIRCUITS cie igse physics
- 2. • State that there are positive and negative charges • Objects can be given one of two types of electric charge: • Positive • Negative
- 3. • State that positive charges repel other positive charges, negative charges repel other negative charges, but positive charges attract negative charges • When two charged objects are brought close together, there will be a force between those objects Like charges repel; opposite charges attract
- 4. • Describe simple experiments to show the production of electrostatic charges by friction and to show the detection of electrostatic charges • Balloons and Static Electricity (colorado.edu)
- 6. • Explain that charging of solids by friction involves only a transfer of negative charge (electrons) • In simple experiments showing the production of electrostatic charges by friction, insulating solids such as plastics are given a charge • This is done using friction to transfer electrons from the surface • By removing electrons, which have negative charge, the insulator is left with a positive charge • Charging of solids by friction involves only a transfer of negative charge (electrons). Positive charge (protons) are trapped inside of the nucleus and cannot be transferred by friction.
- 7. • State that charge is measured in coulombs • the unit for charge, the coulomb. • A coulomb is defined as the amount of charge that passes through an electrical conductor carrying one ampere per second.
- 8. • Describe an electric field as a region in which an electric charge experiences a force • An electric field is a region in which an electric charge experiences a force.
- 9. • State that the direction of an electric field at a point is the direction of the force on a positive charge at that point • A charged object creates an electric field around itself • This is similar to the way in which magnets create magnetic fields • This can be shown by electric field lines • Fields lines always point away from positive charges and towards negative charges Electric fields are always directed away from positive charges and towards negative
- 10. • The direction of the field lines in an electric field is described as: The direction of the force on a positive charge at that point Field lines show the direction that a positive charge would experience if it was at that point
- 11. Electric field lines: • show the path a small positive test charge would take • point from positive charges to negative charges • are at right angles to the surface of a conductor • are more closely packed when the field is stronger
- 12. • Describe simple electric field patterns, including the direction of the field: (a) around a point charge (b) around a charged conducting sphere • Charges and Fields 1.0.60 (colorado.edu)
- 13. Field Lines Around a Point Charge • The electric field is the region in which another charge will experience a force • Fields lines always go away from positive charges and towards negative charges – they have the same direction as the direction of the force on a positively charged particle at a point in that field Electric fields are always directed away from positive charges and towards negative charges
- 14. Field Lines Around a Charged Conducting Sphere • The field lines around a charge conducting sphere are symmetrical, as with a point charge • This is because the charges on the surface of the sphere will be evenly distributed • The charges are the same, so they repel • The surface is conducting, allowing them to move
- 15. (c) between two oppositely charged parallel conducting plates (end effects will not be examined) Field Lines Between Two Oppositely Charged Parallel Conducting Plates • The electric field between two parallel plates is a uniform electric field • The field lines are: • Directed from the positive to the negative plate • Parallel • Straight lines
- 16. • Describe an experiment to distinguish between electrical conductors and insulators • Circuit Construction Kit: DC (colorado.edu)
- 17. Investigating Conductors & Insulators Conductors, Insulators & Electrons • The key difference between conductors and insulators is that: • Conductors allow charge carriers to freely move • Insulators do not allow charge carriers to move • The reasons for this are to do with their internal structure
- 18. Conductors • A conductor is a material that allows charge (usually electrons) to flow through it easily • Examples of conductors are: • Silver • Copper • Aluminium • Steel • Conductors tend to be metals Different materials have different properties of conductivity
- 19. • Recall and use a simple electron model to explain the difference between electrical conductors and insulators and give typical examples • On the atomic scale, conductors are made up of positively charged metal ions with their outermost electrons delocalised • This means the electrons are free to move
- 20. Electrical Conduction in Metals • In a metal, current is caused by a flow of electrons
- 22. • Describe electrical conduction in metals in terms of the movement of free electrons • Metals conduct electricity very well because: • Current is the rate of flow of charged particles • So, the more easily electrons are able to flow, the better the conductor
- 23. Insulators • An insulator is a material that has no free charges, hence does not allow the flow of charge through them very easily • Examples of insulators are: • Rubber • Plastic • Glass • Wood • Some non-metals, such as wood, allow some charge to pass through them • Although they are not very good at conducting, they do conduct a little in the form of static electricity • For example, two insulators can build up charge on their surfaces. If those surfaces touch, this would allow that charge to be conducted away
- 24. • Know that electric current is related to the flow of charge. Define electric current as the charge passing a point per unit time; recall and use the equation: I= Q/t • The current is the amount of charge passing a point in a circuit every second • (It is helpful to think of current as the charge per second) • Charge, current and time are related by the following equation: • Where the symbols: • Q stands for charge (measured in coulombs, C) • I stands for current (measured in amps, A)
- 25. • When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors • This flow of charge is called an electric current • The greater the flow of charge, the greater the electric current
- 26. • Know the difference between direct current (d.c) and alternating current (a.c.) Direct Current and Alternating Current • Current can be direct current (dc) or alternating current (ac) • In terms of calculations they can be treated in the same way
- 27. • State that conventional current is from positive to negative and that the flow of electrons is from negative to positive • Electrons are negatively charged • This means that the electrons flow from negative to positive • Conventional current, however, is still defined as going from positive to negative
- 28. Direct Current (dc) • Direct current is produced when using dry cells and batteries (and sometimes generators, although these are usually ac) • The electrons flow in one direction only, from the negative terminal to the positive terminal
- 29. Alternating Current (ac) • Alternating current typically comes from mains electricity and generators • It is needed for use in transformers in the National Grid (covered later in this topic) • The direction of electron flow changes direction regularly • A typical frequency for the reversal of ac current in mains electricity is 50 Hz
- 30. • Describe the use of ammeters (analogue and digital) with different ranges • Current is measured using an ammeter • Ammeters should always be connected in series with the part of the circuit you wish to measure the current through • Ammeters measure the amount of charge passing through them per unit time, so the ammeter has to be in series so that all the charge flows through it
- 31. Digital or Analogue? • Ammeters can be either • Digital (with an electronic read out) • Analogue (with a needle and scale)
- 32. Analogue Ammeters • Typical ranges are 0.1-1.0 A and 1.0-5.0 A for analogue ammeters • Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements. They should be checked for zero errors before using • They are also subject to parallax error • Always read the meter from a position directly perpendicular to the scale
- 33. Digital Ammeters • Digital ammeters can measure very small currents, in mA or µA • Digital displays show the measured values as digits and are more accurate than analogue displays • They’re easy to use because they give a specific value and are capable of displaying more precise values • However digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down • Digital ammeters should be checked for zero error • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results
- 34. • Know that the current at every point in a series circuit is the same • Circuit Construction Kit: DC (colorado.edu)
- 35. Current in Series Circuits • In a circuit that is a closed- loop, such as a series circuit, the current is the same value at any point • This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part • This means that all components in a closed-loop have the same current The current is the same at each point in a closed-loo
- 36. • The amount of current flowing around a series circuit depends on two things: • The voltage of the power source • The resistance of the components in the circuit
- 37. • Increasing the voltage of the power source drives more current around the circuit • So, decreasing the voltage of the power source reduces the current Current will increase if the voltage of the power supply increases
- 38. • Increasing the number of components in the circuit increases the total resistance • Hence less current flows through the circuit Current decreases if the number of components increases (because there will be more resistance)
- 39. • State that, for a parallel circuit, the current from the source is larger than the current in each branch • A parallel circuit consists of two or more components attached along separate branches of the circuit
- 40. • The advantages of this kind of circuit are: • The components can be individually controlled, using their own switches • If one component stops working the others will continue to function • In a parallel circuit, the current splits up - some of it going one way and the rest going the other • This means that the current in each branch will be smaller than the current from the power supply
- 41. • Recall and use in calculations, the fact that: (a) the sum of the currents entering a junction in a parallel circuit is equal to the sum of the currents that leave the junction Current is split at a junction into individual branches
- 42. • Explain that the sum of the currents into a junction is the same as the sum of the currents out of the junction • At a junction in a parallel circuit (where two or more wires meet) the current is conserved • This means the amount of current flowing into the junction is equal to the amount of current flowing out of it • This is because charge is conserved
- 43. • Note that the current does not always split equally – often there will be more current in some branches than in others • The current in each branch will only be identical if the resistance of the components along each branch are identical
- 44. • Define electromotive force (e.m.f) as the electrical work done by a source in moving a unit charge around a complete circuit Know that e.m.f ismeasured in volts (V) • The electromotive Force (e.m.f.) is the name given to the potential difference of the power source in a circuit • It is defined as The electrical work done by a source in moving a unit charge around a complete circuit • Electromotive force (e.m.f.) is measured in volts (V)
- 45. • Recall and use the equation for e.m.f E= W/Q • The definition of e.m.f. can also be expressed using an equation • Where • E = electromotive force (e.m.f.) (V) • W = energy supplied to the charges from the power source (J) • Q = charge on each charge carrier (C) Note: in circuits the charge carriers are electrons • This equation should be compared to the definition of potential difference (below) as the two are closely related
- 46. • Define potential difference (p.d) as the work done by a unit charge passing through a component Know that p.d between two points is measured in volts (V) • The potential difference between two points in a circuit is related to the amount of energy transferred between those points in the circuit • As charge flows around a circuit energy is transferred from the power source to the charge carriers, and then to the components • This is what makes components such as bulbs light up
- 47. • Potential difference is defined as • The work done by a unit charge passing through a component • Potential difference is measure in volts (V)
- 48. • Recall and use the equation for p.d V= W/Q • The definition of p.d. can also be expressed using an equation • Where • V = potential difference (p.d.) (V) • W = energy transferred to the components from the charge carriers (J) • Q = charge on each charge carrier (C) • In circuits the charge carriers are electrons • This equation should be compared to the definition of e.m.f. as the two are closely related due to conservation of energy
- 49. • Describe the use of voltmeters (analogue and digital) with different ranges • Potential difference is measured using a voltmeter, which can be either • Digital (with an electronic read out) • Analogue (with a needle and scale)
- 50. • Voltmeters are connected in parallel with the component being tested • The potential difference is the difference in electrical potential between two points, therefore the voltmeter has to be connected to two points in the circuit
- 51. Analogue or Digital? • Analogue voltmeters are subject to parallax error • Always read the meter from a position directly perpendicular to the scale • Typical ranges are 0.1-1.0 V and 0-5.0 V for analogue voltmeters although they can vary • Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements • They should be checked for zero errors before using
- 52. • Digital voltmeters can measure very small potential differences, in mV or µV • Digital displays show the measured values as digits and are more accurate than analogue displays • They’re easy to use because they give a specific value and are capable of displaying more precise values • However digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down • Digital voltmeters should be checked for zero error • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results
- 53. ·Calculate the combined e.m.f of several sources in series ·Recall and use in calculations, the fact that: (b) the total p.d across the components in a series circuit is equal to the sum of the individual p.d.s across each component (c) the p.d across an arrangement of parallel resistances is the same as the p.d across one branch in the arrangement of the parallel resistances • Circuit Construction Kit: DC (colorado.edu)
- 54. Potential Difference in Series Circuits • When several cells are connected together in series, their combined EMF is equal to the sum of their individual EMFs The total EMF of these cells is equal to the sum of their individual EMFs
- 55. Potential Difference in Series Circuits • In a series circuit, the sum of potential differences across the components is equal to the total EMF of the power supply In a series circuit the components share the EMF of the power supply
- 56. Potential Difference in Parallel Circuits • A parallel circuit consists of two or more components attached along separate branches of the circuit • • The advantages of this kind of circuit are: • The components can be individually controlled, using their own switches • If one component stops working the others will continue to function
- 57. • The potential difference across each component connected in parallel is the same • This is the opposite of the current, which is different in each branch
- 58. • Recall and use the equation for resistance R= V/ I • Resistance is the opposition to current • For a given potential difference, the higher the resistance, the lower the current • Therefore resistors are used in circuits to control the current • The unit of resistance is the ohm, represented by the Greek symbol omega Ω
- 59. Ohm's Law • The definition of resistance can be given using the equation • Where • R = resistance (ohms, Ω) • V = potential difference (volts, V) • I = current (amperes, A) • Ohm's Law can be stated in words: Current is directly proportional to potential difference as long as the temperature remains constant
- 60. Consequences of Ohm's Law • Resistors are used in circuits to control either • The current in branches of the circuit (through certain components) • The potential difference across certain components • This is due to the consequences of Ohm's Law • The current in an electrical conductor decreases as its resistance increases (for a constant p.d.) • The p.d. across an electrical conductor increases as its resistance increases (for a constant current)
- 61. • Describe an experiment to determine resistance using a voltmeter and an ammeter and do the appropriate calculations • https://learning.cambridgeinternational.org/cla ssroom/course/view.php?id=2987 • Worksheets
- 62. • State, qualitatively, the relationship of the resistance of a metallic wire to its length and cross-sectional area
- 63. • Recall and use the following relationship for a metallic electrical conductor: (a) resistance is directly proportional to length (b) resistance is inversely proportional to cross-sectional area Resistance of a Wire • As electrons pass through a wire, they collide with the metal ions in the wire • The ions get in the way of the electrons, resisting their flow
- 65. Proportionality Relationships for Electrical Conductors • The relationship between resistance, length and cross-sectional area can be represented mathematically • Resistance is directly proportional to length • Resistance is inversely proportional to cross-sectional area (width, or thickness)
- 67. • If the wire is longer, each electron will collide with more ions and so there will be more resistance: The longer a wire, the greater its resistance • If the wire is thicker (greater diameter) there is more space for the electrons and so more electrons can flow: The thicker a wire, the smaller its resistance
- 68. • Sketch and explain the current–voltage graphs for a resistor of constant resistance, a filament lamp and a diode I-V Graphs for Ohmic Resistors, Filament Lamps & Diodes • As the potential difference (voltage) across a component is increased, the current in the component also increases • The precise relationship between voltage and current can be different for different types of components and is shown by an IV graph:
- 69. • The IV graph for a resistor is very simple: The current is proportional to the potential difference • This is because the resistor has a constant resistance
- 70. • For a lamp the relationship is more complicated: • The current increases at a proportionally slower rate than the potential difference • This is because: • The current causes the filament in the lamp to heat up • As the filament gets hot, its resistance increases • This opposes the current, causing it to increase at a slower rate
- 71. • A diode is a non-ohmic conductor that allows current to flow in one direction only • The direction is shown by the triangular arrow of the diode symbol • This is called forward bias • In the reverse direction, the diode has very high resistance, and therefore no current flows • This is called reverse bias • The I–V graph for a diode has a unique shape • When the diode is in forward bias, the graph shows a sharp increase in voltage and current (on the right side of the graph) • When the diode is switched around, in reverse bias, the graph shows a flat line where current is zero at all voltages (on the left side of the graph)
- 72. Resistors in Series & Parallel • Circuit Construction Kit: DC (colorado.edu)
- 73. • Calculate the combined resistance of two or more resistors in series Resistors in Series • When two or more components are connected in series: • The combined resistance of the components is equal to the sum of individual resistances When several components are connected in series, their combined resistance is equal to the sum of their individual resistances
- 74. • State that the combined resistance of two resistors in parallel is less than that of either resistor by itself Resistors in Parallel • When resistors are connected in parallel, the combined resistance decreases and is less than the resistance of any of the individual components • If two resistors of equal resistance are connected in parallel, then the combined resistance will halve The above resistors will have a combined resistance of 2 Ω − half the value of each resistor
- 75. • Calculate the combined resistance of two resistors in parallel Determining Resistance in Parallel • To determine the combined resistance of any combination of two resistors, you must use the equation: